It has been recently attempted intensively to apply organic electroluminescence (EL) elements in a display panel by matrix configuration. A simple matrix method is known as a driving method of this organic EL display panel.
In this system, anodes and cathodes are arranged in a matrix shape, and light emitting elements are disposed at intersections of anodes and cathodes. According to this method, the cathodes are scanned and driven at specific time intervals, and an anode of a desired light emitting element is driven in synchronism therewith, so that the specific light emitting element is selected to emit light.
FIG. 11 is an equivalent circuit diagram showing this simple matrix driving system.
As shown in FIG. 11, anode wires (A1, A2, . . . , Am) and cathode wires (C1, C2, . . . , Cn) are arranged in a matrix shape. Light emitting elements are disposed at intersections of the anode wires and cathode wires.
An example of operation for selecting and lighting L1,1, L2,1, of multiple organic EL elements L1,1, to Lm,n shown in FIG. 11 is described below.
Anode wires A1, A2 are connected to current sources J1, J2 through switches SA1, SA2, respectively. Cathode wire C1 is connected to the ground potential through a switch SC1. By these connections, L1,1, L2,1 are selectively provided with a forward bias voltage, and emit light. At this time, switches SA3 to SAm connect anode wires A3 to An corresponding to these switches to the ground potential, and switches SC2 to SCn connect cathode wires C2 to Cn corresponding to these switches to the Vcc potential. The switches SA3 to SAm and switches SC2 to SCn operate to prevent error of lighting non-selected elements.
Conventionally, when driving the display panel of such simple matrix system, it is a known problem that the anode voltage of the element to emit light is not raised promptly due to capacitive component of the organic EL element. To solve this problem, a driving method disclosed in Japanese Laid-open Patent No. 9-232074 is known. In this driving method, every time the cathode wire is driven, all cathodes are connected to the reset voltage at the same potential, so that the element accumulated charge is instantly discharged to zero.
However, this conventional driving method had the following problems. FIG. 12 is a diagram showing a discharge current waveform in the case of discharge of accumulated charge of a display panel in a configuration of 256×64 dots. By simple matrix driving, all elements are driven in non-luminescent state. An inverse bias charge is accumulated in organic EL elements on the cathode wires except for driven cathode wires. Consequently, by connecting the anode wires A1 to A256 and cathodes C1 to C64 to the ground potential, the accumulated charge in the organic EL elements is discharged. FIG. 12 shows the discharge current waveform at this time. In FIG. 12, the wires are connected to the ground potential at the timing of T1. By this connection, discharge is started. On the actual display panel, there are wiring impedance and output impedance of switching means. Therefore, as shown in FIG. 12, the discharge current of the element accumulated charge shows a gradual approach to zero with the passing of the time. A sufficient discharge time was needed until the element accumulated charge would decrease to a practically safe level. However, such discharge time of accumulated charge was not taken into consideration in the conventional driving method.
Besides, as a result of studies by the present inventor, it was found out that another problem is caused by parasitic capacity of organic EL element. For example, it occurs in the driving circuit shown in the driving method disclosed in Japanese Laid-open Patent No. 6-301355. FIG. 13 is an example of a driving circuit presented in an embodiment of the invention disclosed in Japanese Laid-open Patent No. 6-301355. As shown in FIG. 13, this driving circuit is mainly composed of organic EL elements indicated by diode symbols, anode wires Y1 to Ym, and cathode wires X1 to Xn.
In this driving circuit, suppose the following case:
As a first action, all elements on the cathode wire X1 are driven in non-luminescent state;
As a second action, cathode wire scanning and driving is advanced by one line, and all elements on X2 emit light.
In the first action, all bipolar transistors 101 to 10m are turned off, and the anode wires Y1 to Ym are at the ground potential. A field effect transistor 71 of a row selection changer 8 is turned on, and the cathode wire X1 is connected to the ground potential. Other cathode wires X2 to Xn are turned off except for the field effect transistor 71 of the row selection changer 8, and are pulled up to a forward bias driving voltage VB. Therefore, the organic EL elements on cathode wires X2 to Xn are inversely biased, and an electric charge is accumulated.
In the second action, field effect transistors 111 to 11m are turned off, bipolar transistors 101 to 10m are turned on, and a driving voltage VB is applied to anode wires Y1 to Ym. A field effect transistor 72 is turned on, and cathode wire X2 is connected to the 20 ground potential. Other cathode wires X1, X3 to Xn are turned off except for the field effect transistor 72, and are pulled up to a forward bias driving voltage VB.
Paying attention to cathode wires X3 to Xn in this second action, an electric charge is accumulated in the elements on cathode wires, and a driving voltage VB is generated at both ends of the element. Accordingly, the sum potential 2 VB of the driving voltage VB applied to the anode wires Y1 to Ym and the voltage VB produced by accumulated charge is instantly applied to both ends of the element. Later, the accumulated charge is discharged through a pull-up resistance Rc. Along with this discharge the voltage at both ends of the element gradually approaches the voltage VB. Thus, by the accumulated charge, a maximum voltage of 2 VB is generated at both ends of the element. This maximum voltage 2 VB is also applied to the field effect transistors for driving the cathodes. In these field effect transistors and other semiconductor switching elements, the maximum value of applicable voltage is determined as the absolute maximum rating, individually. If a larger voltage is applied, the reliability of the semiconductor switching element is lowered significantly. It is hence necessary to select a semiconductor switching element having a sufficient withstand voltage for actual voltage. Generally, to heighten the withstand voltage of the semiconductor switching element, it is considered in the semiconductor process, or in the design of the semiconductor, or in both. The higher the withstand voltage, the higher is the cost of the semiconductor switching element, and the scale of integration of elements is lower. Therefore, the conventional device was a serious problem for lowering the cost and reducing the size and weight.
Thus, in the conventional driving method, no particular consideration is given to the discharge time of the element accumulated charge. Accordingly, the anode voltage of the element to emit light is not always raised to high voltage promptly. Besides, an excessively long discharge time is effective as measure against the problem by the element accumulated charge. However, if the discharge time is excessively long, since light is not emitted in the discharge time, the driving efficiency is worsened. By poor driving efficiency, it appears that the display luminance is lowered.